A rare earth silicate (Ln2SiO5:A in which Ln is Y, Gd and/or Lu; and A is Ce, Sm, Eu, Tb and/or Zr) is known as a stimulable phosphor [for example, as described in J. Phys. D: Appl. Phys., vol. 24 (1991), pp. 997–1002]. When the stimulable phosphor is exposed to radiation or ultraviolet rays (primary excitation), the phosphor absorbs and stores a portion of the energy of radiation or ultraviolet rays. The stimulable phosphor then emits stimulated light in the visible wavelength region when exposed to electromagnetic wave such as visible light or infrared rays (secondary excitation). The stimulable phosphor is practically utilized in a radiation image recording and reproducing method, and JP-2,00,696A and JP-3,290,497B propose use of the rare earth silicate in the method as a phosphor of a radiation image storage panel (which is also referred to as “imaging plate”).
The radiation image recording and reproducing method employs a radiation image storage panel containing an energy-storing phosphor such as the stimulable phosphor. A typical process of the method comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the phosphor with a stimulating light (such as a laser beam) to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus processed is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
JP-2001-255,610A discloses a radiation image forming method, which is an alternative process of the radiation image recording and reproducing method. The stimulable phosphor of the storage panel used in the conventional recording and reproducing process plays both roles of radiation-absorbing function and energy-storing function. However, those two functions can be separated in the process. In the process, a radiation image storage panel comprising the stimulable phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising a different phosphor which absorbs radiation and spontaneously emits ultraviolet or visible light. This process comprises the steps of causing the radiation-absorbing phosphor of the phosphor screen to absorb and convert the radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storing phosphor (i.e., stimulable phosphor) of the storage panel to store the energy of the converted light as radiation image information; sequentially exciting the stimulable phosphor with a stimulating ray to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals, whereby giving a visible radiation image.
In the radiation image recording and reproducing method (and in the radiation image forming method), it is required to obtain a clear image with a small dose of radiation. In consideration of this requirement, the radiation image storage panel (or the phosphor screen) preferably has a phosphor layer in which the phosphor is so densely packed that the radiation is efficiently absorbed. Accordingly, the phosphor preferably has a high true density.
It is known that lutetium silicate has a high true density (7.4 g/cm2) and that a silicate phosphor comprising lutetium has a high melting point (higher than 2,000° C.). Accordingly, the silicate phosphor has been hitherto prepared by the steps of melting starting materials at a very high temperature (above 2,000° C.) and gradually drawing up a single crystal of the phosphor from the melt. It is, therefore, not easy to prepare the lutetium silicate phosphor, and hence it is desired that a rare earth silicate phosphor having a high true density, particularly a silicate phosphor containing a heavy rare earth such as lutetium, be more easily prepared.
The rare earth silicate phosphor is also known to absorb radiation such as X-rays and instantly emit a light in the visible wavelength region (instant emission). Because of this property, it is suggested in Nuclear Instruments and Methods in Physics Research A, vol. 416(1998), pp. 333; IEEE Transaction on Nuclear Science, vol. 41(1994), No. 4, pp. 689; and ibid., vol. 47(2000), No. 6, pp. 1781, that the rare earth silicate phosphor be utilized as a scintillator, which is required to comprise phosphor packed densely enough to absorb radiation efficiently.
J. Phys. D: Appl. Phys., vol. 24 (1991), pp. 997–1002 describes preparation of Y2SiO5: (Ce, Sm) phosphor. The disclosed phosphor is prepared by a solid phase reaction method which comprises the steps of mixing Y2O3, CeO2, Sm2O3, SiO2 and NH4 (flux) and firing the mixture.
JP-2-300,696A discloses a stimulable phosphor represented by the formula: YxLuyGdzSiO5:aA,bB in which x, y and z are numbers satisfying the conditions of x+y+z=2, 0<x, 0≦y, 0≦z; A is Ce and/or Tb; B is Zr and/or Sm; and a and b are numbers satisfying the conditions of 2×10−5<a<0.02 and 2×10−5<b<0.02, respectively. The disclosed phosphor is prepared by a sol-gel method comprising the steps of: dissolving in diluted nitric acid Y2O3 and Lu2O3, CeO2 and/or TbO2, and oxide or nitrate of Zr and/or Sm; adding an alcohol and a tetraethyl orthosilicate to the obtained solution and then mixing them completely; adding diluted aqueous ammonia to the mixture to obtain a gel; and heating the gel at a temperature of 1,400 to 1,600° C.
JP-3,290,497B discloses a stimulable phosphor represented by the formula: Y2-xLnxSiO5.yM:zAc in which Ln is at least one rare earth element selected from the group consisting of Y, Gd and Lu; Ac is at least one element selected from the group consisting of Eu, Ce, Sm and Zr; M is at least one element selected from the group consisting of Al and Mg; and x, y and z are numbers satisfying the conditions of 0<x≦2, 0<y≦1.0 and 0<z≦0.1, respectively. The disclosed phosphor is prepared by a solid phase reaction method which comprises the steps of: mixing Y2O3 and/or Lu2O31SiO2 and an oxide of Ac; adding AlF3 and/or MgF2 to the mixture, and then mixing them again; and firing the obtained mixture.
IEEE Transaction on Nuclear Science, vol. 47(2000), No. 6, pp. 1781 discloses a process for preparation of lutetium silicate phosphor. The process comprises the steps of reacting lutetium metal with isopropanol to prepare a alkoxide and firing the alkoxide to obtain the lutetium silicate. According to this process, a phosphor giving high performances when used as a scintillator can be obtained by firing at 1,200° C. However, it is necessary in the process to use mercury, which is an undesirable substance from the environmental viewpoint.